Human observers are quite good at detecting changes in the speed of a single moving object. However, there are usually several objects moving at different speeds in natural situations.Thus we wanted to explore a more complex situation where several objects are moving while the observer is either fixating or smoothly pursuing a target spot.

Two vertically oriented sinewave gratings were moving horizontally at a constant speed, called the pedestal speed. The gratings were 27 deg wide and 9 deg high and were separated vertically by a 2 deg gap. In this gap between the gratings a small fixation spot was either stationary or moved in the same direction as the gratings. During the movement one of the gratings changed its speed for 500 ms. Two different experiments were performed. In a localization task, the subject had to indicate whether the top or bottom grating changed speed. In a detection task, the subject had to indicate in which one of two intervals the speed change took place.

While thresholds for detecting speed changes were in the normal range of Weber fractions of 10%–15%, localization thresholds were dramatically increased to Weber fractions of about 30%–40% whenever there was retinal motion due to differences in eye and pedestal speed. This effect was particularly pronounced when the retinal motion was mainly due to the pedestal motion, and less when it was mainly due to the pursuit eye movements.

We conclude that localization of speed changes is exceedingly difficult. This is probably due to the dominance of relative motion signals when several objects are moving independently. Smooth pursuit is an effective means to improve performance under these conditions.